The speaker starts off right away by letting us know that he's specifically talking about America as a perishing republic that's going to rot because it's become too much of an empire. And just like every other empire, this one will decay like a fruit to (metaphorically) return to mother earth and begin the cycle all over again. He then looks to the actual people that are causing the decay and says, "Hey, I don't blame you. Life is good and all things must pass." But there is a glimmer of hope when he tells the republic to shine on despite the decay.
As far as his children are concerned, he'd prefer to keep them out of the whole mess, maybe hide them out in the mountains someplace where it's safe from the monster that is the decaying empire of America. After all, when the cities are laid to waste, those mountains remain. He then reminds the boys not to get caught up in the whole love of mankind thing because man ain't nothing but an "insufferable master." If the kids do decide to get tangled up in the mess, the speaker warns them that they'd be doing what other noble spirits have done (like God) who walked the earth and got caught in the trap, only to suffer and die. Happy ending? We think not.